I have used the instructions in this page to install amd64 OpenBSD
6.3 with the xfce4 desktop and a range of applications on a Thinkpad
X220 (legacy boot) and a Thinkpad X61s. I have also installed the 32bit
version of OpenBSD 6.3 on a Thinkpad T42 with the radeon graphics chip
and 1Gb of RAM - it runs fine but slowly.

Set up a package mirror and install a
package

The pkg_add command reads the URL of the package
mirror from the /etc/installurl file.

If you installed the OpenBSD package sets from the Internet, you will
already have the/etc/installurl file in place and you can
skip this section. If, like me, you prefer to install OpenBSD from the
install.iso or install.fs images, you will
need to create the /etc/installurl file as below...

As root add some lines to
/etc/rc.conf.local to enable power management
(apmd) so that you can use Fn-F4 to suspend your thinkpad,
and to enable the graphical log-in manager xenodm. Xenodm
is an OpenBSD fork of the venerable xdm.

Then as user add an .xsession file with
a line that will start consolekit so that you can shutdown &c from
within xfce4.

$ cat .xsession
exec ck-launch-session startxfce4

Memory limits in /etc/login.conf

My Thinkpad X220 has 8Gb of RAM. Firefox (and Chromium) like lots of
RAM. By default, OpenBSD enforces limits on the largest amount of RAM
that a user level program can use to around 1.5Gb, and if it exceeds
that limit, the OOM killer will terminate the process. Your user will be
part of the staff group and you can edit /etc/login.conf to increase the default limit. Mine is set as follows...

Once mounted, you can use a graphical file manager like Thunar to
copy and paste files to and from your storage stick. You can't unmount
the USB stick from Thunar, remember to use the umount
/dev/sd1i command before removing the USB stick.

Use xfce4-mount-plugin and an /etc/fstab
entry to allow graphical mount/unmount of a USB thumb drive

The xfce4-mount package is installed as part of the xfce4 package set. Add an icon for the plugin to the XFCE4
panel by right-clicking on the panel and selecting
Panel | Add New Items and searching for 'mount'.

By default, xfce4-mount-plugin lists all the devices
including the default local hard drive including all the partitions on
sd0. I can set options to prevent that and to use a custom mount
command. Right click over the xfce4-mount icon and select
Properties | File Systems tab. Add the pattern
/dev/sd0* to the Exclude specified file systems textbox so
the local drive is not listed.

Right-click on the xfce4-mount icon, and selected Preferences
| Commands and write the following in the Custom Commands
textboxes, after ensuring that the Custom Commands checkbox was
ticked...

doas mount %m
doas umount %m

Now to ensure that a USB stick is listed in the
xfce4-mount-popup list, you have to add a line for the device to
/etc/fstab. My extra line looks like this (adapted from the
examples in man fstab...

/dev/sd1i /home/keith/usb msdos rw,noauto 0 0

Using an fstab entry like this means that only one vfat
formatted USB thumb drive will be listed and available with mouse
clicks.

Set up updates (6.2 info, 6.3 has no updates as yet)

The syspatch command provides binary updates to the core
system. Running the syspatch command without arguments
while connected to the Internet resulted in the first three errata for
OpenBSD 6.2 being installed as you can see in the transcript below.

M:Tier sponsors OpenBSD and has provided binpatches for
the packages in the stable release for use together with the
openup script. Their update page has not yet been
updated for OpenBSD 6.3. They recommend using openup to
upgrade packages that you have installed in addition to the base
system.

Install OpenBSD with whole drive encryption (advanced)

In OpenBSD 6.3 I used the commands
detailed in the FAQ to create an encrypted softraid device and then
install OpenBSD within that device. I omitted the command to over-write
the disk with random numbers because of time.

Because I install from a USB stick, the drive numbering is 'off by
one' compared to the example commands in the FAQ, so the fixed disk in
the laptop is sd0 and the USB stick that I am installing from is sd1.
The softraid device I use to install OpenBSD is therefore sd2. The
commands I actually used in the installer shell session are shown
below.

Boot from the installer USB stick and then select
[S]hell to get a command prompt...

The exit command starts the installer script and you
type in the root password, your user and the user password and so on.
Then you select sd2 as the installer disk. You also have to
specify [disk] as the location of the package sets and then
answer 'no' when asked if the disk is mounted, select sd1,
mount the a partition. The installer script should then
find the package sets.

Simple wifi script

There is no graphical wifi manager available on OpenBSD. I like this
little script that runs from my user account and simply automates the
typing in of the ifconfig commands. None of the network
related configuration files are changed so I can always fall back on the
ifconfig commands in a new place. The script itself also
serves as a reminder of the syntax of the commands.